C# in Depth

76 CHAPTER 3 Parameterized typing with genericsI deliberately made RefSample a struct (and therefore a value type) to emphasize thedifference between the constrained type parameter and the type itself. RefSample is still a value type with value semantics everywhere—it just happens to usethe string type wherever T is specified in its API.When a type parameter is constrained this way, you can compare references (includingnull) with == and !=, but be aware that unless there are any other constraints, onlyreferences will be compared, even if the type in question overloads those operators (asstring does, for example). With a derivation type constraint (described in a littlewhile), you can end up with “compiler guaranteed” overloads of == and !=, in whichcase those overloads are used—but that’s a relatively rare situation.VALUE TYPE CONSTRAINTSThis constraint (expressed as T : struct) ensures that the type argument used is avalue type, including enums. It excludes nullable types (as described in chapter 4),however. Let’s look at an example declaration:class ValSample where T : structValid closed types include■■ValSampleValSampleInvalid closed types include■■ValSampleValSampleThis time ValSample is a reference type, despite T being constrained to be a valuetype. Note that System.Enum and System.ValueType are both reference types inthemselves, so aren’t allowed as valid type arguments for ValSample. Like referencetype constraints, when there are multiple constraints for a particular type parameter,a value type constraint must be the first one specified. When a type parameter is constrainedto be a value type, comparisons using == and != are prohibited.I rarely find myself using value or reference type constraints, although we’ll seein the next chapter that nullable types rely on value type constraints. The remainingtwo constraints are likely to prove more useful to you when writing your owngeneric types.CONSTRUCTOR TYPE CONSTRAINTSThe third kind of constraint (which is expressed as T : new() and must be the lastconstraint for any particular type parameter) simply checks that the type argumentused has a parameterless constructor, which can be used to create an instance. Thisapplies to any value type; any nonstatic, nonabstract class without any explicitlydeclared constructors; and any nonabstract class with an explicit public parameterlessconstructor.Licensed to Rhona Hadida